The present invention relates to a control apparatus for a magnetic bearing for rotatably supporting a rotor by using a magnetic floating type rotor, and to a rotating machine using such a control apparatus.
A magnetic bearing according to the invention is an active magnetic bearing for supporting a rotor in a space by an attracting force of electromagnets which face each other through the rotor. To stably support the rotor, the position of the rotor is always monitored by a displacement sensor, and an electromagnetic force of an active type magnetic bearing is controlled on the basis of an output signal of the displacement sensor. A compensating circuit which is used in the control of the magnetic bearing is generally fundamentally based on a PID control.
FIG. 7 shows a concept of a radial bearing as one of the magnetic bearings. Reference numeral 1 denotes a rotor; 2 a displacement sensor; 3a and 3b electromagnets; 4 a converter of the displacement sensor; 5 a compensating circuit; 6 a positive/negative switcher; 7a and 7b power amplifiers (current amplifiers); and 8 an auxiliary bearing. The control of such a magnetic bearing has been disclosed in "Dynamics of Apparatuses Using Electromagnetic Force", Corona Co., Ltd., published on 25 Jul., 1990, pages 159-176, and Hannes Bleuler, "Decentralized Control of Magnetic Rotor Bearing Systems", (thesis for a degree of a Switzerland College of an Institute of Technology), Juris Druck+Verlag Zurich, 1984. An apparatus for providing a countermeasure to an overload at the time of a high speed rotation in case of using the magnetic bearing is disclosed in JP-A-4-8911.
In the above conventional magnetic bearing, in order to assure a rigidity or damping property of the bearing, it is necessary to set the gain of a compensating circuit to a large value. Such a point is particularly remarkable in a high frequency region. In this case, noises cannot help being amplified. On the other hand, since a load of a power amplifier is an electromagnet, namely, inductance, a current control ability is limited in the high frequency region in which the impedance increases. When a current command over the ability is given to the power amplifier, since the ability of the power amplifier is saturated, phase characteristics are deteriorated and it is difficult to control the magnetic bearing as a whole system, and instability is enhanced. To avoid such a situation, therefore, it is necessary to prevent a current command in a high frequency range, such as noises or the like, from being inputted to the power amplifier.
As one of the causes of the high frequency noises, there is a run-out which is captured by a displacement sensor. As a narrow meaning, "run-out" means noises which are generated in a signal of the displacement sensor due to a distortion of the surface shape of the rotor. As a broad meaning, "run-out" means noises which also include a whirling signal which is caused in the signal of the displacement sensor due to a deforming shape of the rotor. In the specification, the former one is called a "run-out". The noises due to the run-out include the noises which are synchronized with the rotational speed and their double harmonics. As the diameter of the machine increases and the machine rotates at a high speed, such a noise problem becomes typical and a countermeasure is required.
An apparatus for attracting a rotary shaft in one direction when an output of the displacement sensor is equal to or larger than a set value has been disclosed in JP-A-4-8911. In such a conventional apparatus, however, there is an inconvenience such that it is necessary to attract the rotary shaft by an auxiliary magnet, and the apparatus increases in size.